1. Field of the Invention
This invention relates to a temperature control apparatus for a furnace having a plurality of mutually interfering zones therein (to be referred to as the "multi-zone furnace", hereinafter) for heat-treating works at different temperatures, which apparatus automatically switches temperature distribution pattern in the furnace and deals with the inter-zone thermal interference. More particularly, the invention relates to a temperature control apparatus for controlling the temperature distribution in the so-called tunnel furnace while dealing with inter-zone interference therein. Such tunnel furnace maintains a specific pattern of temperature distribution in the longitudinal direction thereof so as to heat-treat works while they are moved therethrough by a conveyor or the like, using the temperature control apparatus to automaticalluy switch the pattern of temperature distribution in the furnace when the kind of works being heat-treated is changed.
2. Description of the Prior Art:
In tunnel type furnaces, such as conveyor furnaces and pusher furnaces, a proper temperature distribution pattern is established therein at first and then works are conveyed therethrough at a suitable speed so as to carry out the desired heat treatment.
FIG. 2 schematically shows a conventional tunnel furnace. The furnace 1 has a plurality of zones Z.sub.1, Z.sub.2, . . . , Z.sub.n, and each zone is heated by a heating element 2 such as an electric heater. The heating of the furnace is controlled zone by zone either continuously or intermittently by actuating means 3 such as thyristors or other suitable electromagnetic switches. Heating-portion thermometers 4, such as thermocouples, are disposed in the individual zones so as to detect the actual zone temperatures and communicate the detected temperatures to controllers 28A, 28B, . . . , 28N which are associated with the furnace zones respectively. Each controller has an arithmetic-logic unit 27A which compares the thus communicated actual zone temperature against the set value on a temperature setting element 27 thereof, and the unit 27A generates a control signal 5 based on the result of the comparison.
Each control signal 5 is applied to the actuating means 3 of the associated furnace zone, so as to regulate the energy supply to the heating element 2 of the furnace zone from an energy source 2A until the control signal 5 is reduced to nil, i.e., until the actual zone temperature is brought to the set value of the zone temperature. In cases when the actuating means 3 are thyristors or other electromagnetic switches and the power source 2A is an electric power source, the control signals 5 regulate the making and breaking of the electric circuits through the actuating means 3.
The above temperature control brings about an overall temperature distribution pattern in the furnace 1, such as that of the solid line curve Pa or the dashed-line curve Pb of FIG. 2. Works 6 are heat-treated as they are moved through the furnance 1, as shown by the arrow 30, while the desired temperature distribution pattern is maintained therein.
To check whether the desired temperature distribution pattern is built up by the heating elements 2 in the furnace 1 or not, the example of FIG. 2 uses an intra-furnace thermometer 31 of movable type. The intra-furnace thermometer 31 moves in the furnace 1, so as to detect and communicate the temperature within the furnace 1 to a recorder 32. Whereby, the recorder 32 stores and displays the intra-furnace temperature.
In the conventional arrangement of FIG. 2, when the values and distribution of the temperature recorded are different from what is desired, set values on the setting elements 27 of the related controllers 28A-28N are manually changed, and after the temperature in the furnace is stabilized, the temperature distribution in the furnace 1 is measured again. Since adjacent ones of the sub-divided zones Z.sub.1 -Z.sub.n of the furnace 1 are not completely independent and the furnace 1 has to provide for the passage of the works 6 to be heat-treated, the temperature at controllable portions or heating portions of the furnace is inevitably different from that at the inside or work passage thereof. Accordingly, even when the temperature setting of any particular zone is independently modified to a desired level, the inside temperature of that zone is affected by the adjacent zones on both sides thereof, and the desired inside temperature is not necessarily produced by such a modification of the setting.
In the actual change of the temperature distribution in a furnace by modifying the setting of each heating element 2, it has been a practice to change the setting at first and wait until the inside temperature of the furnace is stabilized, to measure the temperature distribution pattern in the furnace by moving the intra-furnace thermometer 31, and to readjust the setting depending on the difference between the desired temperature in the furnace and the thus measured temperature. The above measurement and the readjustment are repeated until the desired temperature distribution pattern is produced in the furnace.
Accordingly, the conventional temperature control apparatus has a shortcoming in that considerable time and labor are necessary to produce the desired temperature distribution pattern in the furnace 1. The recent trend of industrial manufacturing is to produce a large variety of good in small quantities, and the temperature distribution pattern necessary to meet such trend is diversified. Thus, the above time-consuming and laborious process of the setting readjustment is frequently required.
Further, it requires not only a complicated procedure but also considerable experience and skill to find out and set suitable settings on the controllers 28A-28N for producing the desirable temperature distribution pattern for a specific kind of work 6 and to readjust the setting repeatedly in the above-mentioned manner each time the kind of work is changed.